مُسْنَدُ أَحْمَدَ
Musnad Ahmad 3545
Musnad of Abdullah bin Mas'ood
It was narrated that ‘Abdullah bin Maʼqil bin Muqarrin said: My father and I entered upon 'Abdullah bin Mas'ood (رضي الله عنه) and he said: Did you hear the Prophet (ﷺ) say: "Regret is repentance"? He said: Yes. On one occasion he said: I heard him say, "Regret is repentance."
Musnad Ahmad 3568Saheeh; this is a hasan isnad]